Greyrock Mountain EarthCache
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The aim of this EarthCache is to measure the change in elevation between the Cache la Poudre River and the top of Greyrock Mountain using only your GPS receiver. Parking for the trailhead is in a paved parking area on the south side of the highway. The trailhead begins here and heads across the river, before heading west, then north up the mountain. Take your lower elevation reading on the north side of the river, at N 40 41.729 W 105 17.095.
Greyrock mountain is the visible portion of a formation called the Log Cabin Batholith. A batholith is an intrusion of igneous rock into the existing parent rock. At Greyrock Mountain, the Log Cabin Batholith formed from small plutons of magma that rose through fractures and faults into the existing metamorphic gneiss and schist of the basement rock. The magma cooled slowly, forming large, coarse crystals of granite. This is Silver Plume Granite, a slightly pink granite which was formed approximately 1.4 billion years ago. Over time, the overlying gneiss and schist weathered and eroded, and the granite was exposed. Further erosion created the granite dome which you see here.
To log this cache, complete the following: 1.) Walk the trail to the top of Greyrock Mountain and find the change in elevation from your first set of coordinates near the river.
Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.
The above information was compiled from the following sources:
2007. Chronic H., and F. Williams. Roadside Geology of Colorado, 2nd Ed.
2004. Cole, J.C. Regional Crustal Conditions During Emplacement of 1400 Ma Granites – A Tale Told by Three Plutons, in Symposium on the Geology of the Front Range in honor of William A. Braddock. M. Coates, E. Evanoff, & M. Morgan, Eds.
2004. Hopkins, R.L., and L.B. Hopkins. Hiking Colorado’s Geology
Thanks to the Roosevelt National Forest for allowing placement of this EarthCache!
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